1. Field of the Invention The present invention relates to a TiAl alloy for use at high temperatures, in particular in the range of from 750° C. to 950° C., and to the production and use thereof
2. Discussion of Background Information
Alloys based on intermetallic titanium aluminide compounds are used in the construction of stationary gas turbines or aircraft engines, for example as a material for rotor blades, because they have the mechanical properties required for use and, in addition, possess a low specific weight, so that the use of such alloys can increase the efficiency of stationary gas turbines and aircraft engines.
A large number of TiAl alloys have accordingly already been developed, there currently being used in particular TiAl alloys based on the intermetallic γ-TiAl phase which are alloyed with niobium and molybdenum or boron and are therefore referred to as TNM or TNB alloys. Such alloys comprise as the main constituent titanium as well as approximately from 40 to 45 at. % aluminum, about 5 at. % niobium and, for example, 1 at. % molybdenum as well as small amounts of boron. The microstructure is characterized by a high γ-TiAl content and likewise significant amounts of α2-Ti3Al, it being possible for further phases, such as, for example, β-phase or B19 phase, to occur in a smaller amount.
The known TNM or TNB alloys based on γ-TiAl usually have a globular, equiaxial γ-TiAl structure, a lamellar structure or a duplex structure with globular, equiaxial γ-TiAl grains and lamellar regions of γ-TiAl and α2-Ti3Al. Although such γ-TiAl alloys, in particular with lamellar microstructures, generally exhibit very good mechanical properties up to 750° C., the mechanical properties are impaired at higher temperatures owing to the thermodynamic instability of the structure, the creep resistance falling in particular. At temperatures above 700° C., oxygen additionally diffuses into the rim zone of the component, resulting in phase instabilities, microstructural changes and embrittlement of the material.
In view of the foregoing, it would be advantageous to have available an alloy which has a low specific weight similar to the known γ-TiAl alloys as well as comparable mechanical properties, in particular at high temperatures, the range of use preferably being enlarged to temperatures in the range of from 750° to 900° C. or 950° C. Such an alloy should be capable of being produced and processed on an industrial scale without an excessive outlay and also of being used reliably in stationary gas turbines and aircraft engines.